feat(scoreboard): the layer is available to ALL scenes (YT_ACTIVITY_TICKER)

Creator ruling: the Scoreboard layer exists in every scene rather than being added five times.
ScoreboardHealer.Ensure keeps exactly one per scene — healed on load like the Background and Brand
logo — so a scene can never end up with two, and 'off' is the ordinary per-scene eye toggle rather
than a delete.

It is appended as the TOPMOST layer on purpose: a full-width strip draws over the scene, and
inserting it above the background would break the Brand logo's documented invariant (directly above
the background). That choice is also why the layer-reorder test still passes untouched.

Also: a layer-list icon for the type, so the row is not blank.

Guard: ScoreboardHealerTests — one per scene, topmost, exact strip geometry, idempotent, and a
duplicate healed back to one.

Controls/LeftPanel.xaml

@@ -208,6 +208,9 @@
                                         <Path Data="M4,5 H20 V19 H4 Z M10,9 L15,12 L10,15 Z" Stroke="{StaticResource Brush.Tertiary}" StrokeThickness="1.6" StrokeLineJoin="Round" Stretch="Uniform">
                                             <Path.Style><Style TargetType="Path"><Setter Property="Visibility" Value="Collapsed"/><Style.Triggers><DataTrigger Binding="{Binding IsMediaSource}" Value="True"><Setter Property="Visibility" Value="Visible"/></DataTrigger></Style.Triggers></Style></Path.Style>
                                         </Path>
+                                        <Path Data="M3,9 H21 V15 H3 Z M6,12 H12 M15,12 H18" Stroke="{StaticResource Brush.Tertiary}" StrokeThickness="1.6" StrokeLineJoin="Round" Stretch="Uniform">
+                                            <Path.Style><Style TargetType="Path"><Setter Property="Visibility" Value="Collapsed"/><Style.Triggers><DataTrigger Binding="{Binding IsScoreboard}" Value="True"><Setter Property="Visibility" Value="Visible"/></DataTrigger></Style.Triggers></Style></Path.Style>
+                                        </Path>
                                         <Path Data="M6,16 V11 A6,6 0 0 1 18,11 V16 L20,18 H4 Z M10,20 H14" Stroke="{StaticResource Brush.Tertiary}" StrokeThickness="1.6" StrokeLineJoin="Round" Stretch="Uniform">
                                             <Path.Style><Style TargetType="Path"><Setter Property="Visibility" Value="Collapsed"/><Style.Triggers><DataTrigger Binding="{Binding IsAlertBox}" Value="True"><Setter Property="Visibility" Value="Visible"/></DataTrigger></Style.Triggers></Style></Path.Style>
                                         </Path>